Handover was traditionally performed within wireless communication systems only when a mobile terminal, such as a mobile telephone, moved from one cellular coverage area to another. To accommodate the transition from one coverage area to another, the base station handling a mobile communication session for the mobile terminal would hand over control of the communication session to the base station of the new coverage area just before the mobile terminal entered the new coverage area. In other words, handover was only performed when deemed necessary to accommodate relocation of the mobile terminal. Some relatively modest amount of overlap of coverage was typically provided between adjacent cells to accommodate a smooth handover without risk of interruption of the communication session, i.e. without dropping the call.
With the proliferation of mobile communication systems, particularly with the advent of so-called “picocells”, “umbrella cells”, etc., there is now much greater overlap of coverage areas. A mobile terminal may now be positioned at a location covered by multiple base stations corresponding to multiple coverage areas. Accordingly, handovers need not be performed only because a mobile terminal is moving out of one discrete coverage area and into another. Rather, handovers can advantageously be performed even if the mobile terminal is stationary. For example, a handover may be performed whenever an improvement in performance can be achieved by switching a communication session from one base station to another. Performance can be a fairly complicated function of a variety of signalling parameters. Typically, though, an improvement in performance is achieved if a communication session can be reliably accommodated using less power by a different base station.
Accordingly, a wireless communication system can evaluate the performance gain, if any, that can be achieved by switching from the base station currently controlling a communication session of the mobile terminal and any other base stations having coverage areas overlapping the location of the mobile terminal. A handover is then made to the base station providing the greatest performance gain. By ensuring that each communication session is controlled by the base station providing the best performance, overall system resources are thereby conserved so as to improve overall system bandwidth to accommodate a greater number of calls at any given time. To prevent the system from frequently switching a given communication session back-and-forth between overlapping base stations due to slight variations in relative performance, a minimum “hysteresis” margin may be employed. A handover is only performed if the expected performance gain exceeds the margin.
Although the use of performance gain to determine whether to perform a handover is useful, considerable room for improvement remains. In particular, current communication systems do not explicitly take into account all aspects associated with a handover. In some cases, although a performance gain can be achieved with the handover, the overall situation after the handover might not yet be optimal. The aforementioned hysteresis margin is merely a minimum threshold provided to prevent frequent back-and-forth switching between base stations. Hence, although the performance gain may exceed the hysteresis threshold thus triggering a handover, a net loss of resources might result.
An optimized handover approach is particularly problematic within “Beyond Third Generation” (B3G) communication systems, which simultaneously employ different radio access technologies (RATs) such as Global System for Mobile Communications (GSM) technologies, Universal Mobile Telecommunications Systems (UMTS) employing wideband code-division multiple access (WCDMA) technologies, Super Third Generation (S3G) technologies, Wireless Local Area Network (WLAN) technologies, and Worldwide Interoperability for Microwave Access (WiMAX) technologies. Within B3G systems, a mobile terminal may be located within overlapping coverage areas employing entirely different RATs, as shown in FIG. 1, each having their own controllers interconnected to one another via a Multi-Radio Network Controller (MRNC). The overlapping coverage areas might also include coverage areas using the same RAT but administered by different operators, as shown in FIG. 1 by way of GSM A and GSM B. The problems associated with performing a resource-efficient handover between different RATs or between the same RAT administered by different operators can be significant. These problems can be even more significant if the mobile terminal is also in motion, particularly at high speed perhaps within an automobile or train, because high speed movement may trigger frequent handovers as the mobile terminal passes through various coverage areas, remaining in some areas only briefly.
Within a B3G system, the handover of a mobile terminal from one RAT to another is controllable by a Multi-Radio Resource Management (MRRM) function. Some of the problems associated with performing handovers between different RATs are discussed on “Handover Between WCDMA and GSM”, Ericsson Review, No. 1, pages 6-11, 2003. See also U.S. Patent Application 2002/0160785, of Ericsson, entitled “Commanding Handover between Differing Radio Access Technologies” and U.S. Patent Application 2004/0090937 of Nokia, entitled “Method and Apparatus for Performing Inter-Technology Handoff from WLAN to Cellular Network.” However, these documents do not set forth techniques that adequately address the above problems associated with handovers between different RATs.
Accordingly, there is a need to provide techniques that efficiently trigger handovers, particularly within B3G systems, and it is to that end that the present invention is primarily directed.